The invention relates to a novel use of water-soluble polyvinyl alcohol (co)polymers, to an electrodeposition bath comprising polyvinyl alcohol (co)polymers, and to coated substrates produced using same.
Electrodeposition coating is a well-known method of coating the surface of electrically conducting articles (compare, for example, Glasurit Handbuch Lacke und Farben, Curt R. Vincentz Verlag, Hanover, 1984, pages 374 to 384 and pages 457 to 462, and also DE-A-35 18 732, DE-A-35 18 770, EP-A-0 040 090, EP-A-0 012 463, EP-A-0 259 181, EP-A-0 433 783 and EP-A-0 262 069). The method is used to coat objects made of metal, especially for the priming of automobile bodies, or else to coat conductive plastics.
The coating materials used in electrodeposition coating generally comprise amino or carboxyl-containing synthetic resin binders, with dispersibility in water being achieved by neutralization of the amino or carboxyl groups. The electrodeposition coating materials may further include special grinding resins and possibly further constituents not dispersible in water, such as polymers, plasticizers, pigments, fillers, additives, and auxiliaries. The crosslinking agents used in the electrodeposition coating materials either are not dispersible in water or may be water-dispersible, with the electrodeposition coating materials being externally crosslinking or else self-crosslinking, or being curable with condensation.
Modification to the binders, selection of the crosslinkers, and variation of the composition of the ingredients of the electrodeposition coating material influence the properties of the coating, such as corrosion protection, adhesion, and leveling, for example. For instance, there have been disclosures in particular of electrodeposition coating materials where by adding polymer microparticles or suspended and/or dispersed polymer powders the intention is to exert a favorable influence on corrosion protection, especially at edges, [lacuna] on leveling.
For instance, EP-A-0 259 181 recommends remedying the increased susceptibility to corrosion which is observed at edges of the coated substrate and is caused by a paint film of insufficient thickness by adding polymer microgels, possible ingredients of such microgels being, for example, poly(meth)acrylate copolymers in combination with ethylenically unsaturated vinyl compounds.
Microgel dispersions which are based on epoxy-amine adducts and can be added subsequently are notable for their high compatibility and efficacy as edge protection additives, as described in EP 0 626 000.
DE-B-26 50 611, EP-A-0 052 831, DE-A-39 40 782, EP-A-0 433 783, SU-A-436890, JP-A-53094346, JP-A-79028410 and JP-A-0624820 describe electrodeposition coating compositions with suspendable or dispersible polymer powders which are predominantly free from ionic groups, are able to melt on baking if desired, and are uncrosslinked or crosslinked, said coating compositions further comprising the water-dispersible synthetic resins that are typical of electrodeposition coatings. The particle sizes of such polymer powders may considerably exceed the particle sizes of the water-dispersible synthetic resins of known electrodeposition coating materials: the average particle diameter in JP-A-0624820 is from 1 to 50 micrometers and in DE-A-39 40 782 or EP-A-0 433 783 is from 0.1 to 100 micrometers.
In many cases, the addition of the polymer particles described in EP-A-0 259 181, DE-B-26 50 611, EP-A-0 052 831, EP-A-0 433 783, SU-A-436890, JP-A-53094346, JP-A-79028410 and JP-A-0624820 to aqueous electrodeposition coating materials leads to an improvement in edge coverage. On the other hand, despite the improved edge coverage, the corrosion protection afforded by the deposited electrodeposition coating films, especially at the edges, is inadequate.
Disadvantageous side effects of adding polymer powders include a deterioration in the throwing power of the electrodeposition coating materials and in adhesion to the substrate and/or to subsequent coatings, such as paint films applied subsequently or PVC underbody protection, impairment of the mechanical properties, such as flexibility, stretchability, fracture strength and impact strength, poorer flow properties, and a drastic deterioration in leveling.
A furthermore [sic] key disadvantage of the aqueous and nonaqueous formulations described in the patents EP-A-0 259 181, DE-B-26 50 611, EP-A-0 052 831, EP-A-0 433 783, SU-A-436890, JP-A-53094346, JP-A-79028410, JP-A-0624820, SU-A-661637, SU-A-998592 and SU-A-310952 is the inadequate stability of the coating materials, which tend toward sedimentation. In aqueous electrodeposition coating materials, this may result in massive coverage of the ultrafiltration membrane with coarse polymer particles.
The stability disadvantages of the coating materials are alleviated by incorporating copolymers having vinyl acetal, vinyl alcohol and ethylene units directly into the resins, and/or by grafting reaction, as described in DE 196 18 379.
In this case a fraction of more than 10% by weight of polymer resin is needed in order to achieve sufficient edge coverage.
The incorporation of polymer powder or microgels requires fractions in the percent range, with a deterioration—in some instances drastic—in leveling.
Significantly more effective, even at low concentrations such as 500 ppm, in the electrodeposition coating material are water-soluble cellulose ethers, such as hydroxyethylcellulose (EP 0 640 700). The activity does not last, since the polymer degrades.
Polyvinyl alcohols are used multifariously in coating materials, in particular as suspension stabilizers for the polymerization of vinyl monomers. Whereas the use of polyvinyl alcohols as complexing agents and suspension stabilizers in the pretreatment of iron, steel, zinc and aluminum sheets, in combination with chromates and/or fluorine compounds, is known (J 73008702, WO 9627034), especially the electrophoretic deposition of metal suspensions, such as aluminum (SU 738334, J-A-111201), metal oxide suspensions, such as of chromium, aluminum, titanium and zirconium oxides (J-A-111201, SU 493817), metal salt suspensions, such as of lead, zinc or copper salts (SU 436890, SU 511392, SU 054452, WO 9208168), and also direct deposition of metals, such as lead (SU 321265), the direct use in electrodeposition coating materials is restricted to subsequent treatment of the deposited film by contact with an aqueous polyvinyl alcohol solution followed by baking. This subsequent treatment achieves a flatting effect (JP 56044799) or reduces surface defects, such as craters (DE 4303787).